In order to deal with interference problems associated with aggressive channel reuse schemes (such as a channel reuse of one), various interference management/avoidance techniques have been proposed for use in wireless networks.
For example, assuming an interference limited system, frequency diversity and interference averaging can be achieved in a network by exploiting orthogonal frequency hopping such as Cyclic Frequency Hopping (CFH).
In a conventional cellular system, considering downlink, users receive interference from other cells. If no frequency hopping is used, certain user equipment will always receive interference at the same frequency from the same base station. If this interference level is low, the quality of the communication for this user will be acceptable. On the other hand, if the user equipment receives severe interference from a base station, then it will experience an outage.
CFH works by bringing up the performance of user equipment (UE) with poor quality links to an average quality level, while bringing down the performance of UEs communicating on high quality links again to this average. Since the performance of the bottleneck users has increased, there are fewer users experiencing outages, whereas the number of UEs experiencing high quality communication decreases. Cyclic and other forms of orthogonal frequency hopping are described in R. L. Pickholtz et al. (“Theory of Spread Spectrum Communications—A Tutorial”, IEEE Trans. Comm. 30(5); 855-884, May 1982).
The fact that it is simple to implement and appropriate for providing robust communications links in interference limited and frequency selective channels, makes CFH the most popular frequency hopping technique in commercial communications systems (e.g. GSM).
Other techniques for dealing with interference require transmission coordination of base stations, which are interferers of each other. However, in many current wireless communication architectures neighboring base stations do not have a wired link between each other. Therefore, information exchange, and hence transmission coordination, is difficult to achieve in a timely fashion among the base stations.
For example, it has been proposed to have an interference management technique called Dynamic Frequency Hopping (DFH) that incorporates a non-traditional Dynamic Channel Allocation (DCA) scheme with slow frequency hopping (Z. Kostic, and N. Sollenberger, “Performance and Implementation of Dynamic Frequency Hopping in Limited-Bandwidth Cellular Systems”, IEEE Transactions on Wireless Communications, Vol. 1, No. 1, January 2002).
The main objective of DFH is to provide capacity improvements through the addition of interference avoidance, which are higher than those provided by conventional frequency hopping, while preserving interference averaging characteristics of conventional frequency hopping in order to provide robustness to changes in interference.
For generic cellular systems, with frequency reuse of one, Wang et al. and Kostic et al. have shown that implementing interference avoidance on top of frequency hopping can result in considerable capacity improvements (Wang et al., “Analysis of Frequency-Hopped Cellular Systems with Dynamic FH Pattern Adaptation”, in Communication Theory Mini-Conference, IEEE Globecom, 1998, Sydney: Kostic et al., “Dynamic Frequency Hopping in Wireless Cellular Systems—Simulations of Full-Replacement and Reduced-Overhead Methods”, in Proceedings of the IEEE VTC'99, vol. 2, pp. 914-918, May, 1999, Houston).
DFH is based on adjusting or creating frequency hopping patterns based on interference measurements. This technique combines traditional frequency hopping with dynamic channel assignment (DCA), where a channel is one frequency in a frequency hop pattern. The continuous modification of frequency hop patterns based on measurements represents an application of DCA to slow frequency hopping. However, the fact that only some subset of frequencies in the whole FH pattern is replaced by a better quality subset makes this a non-traditional DCA scheme. The modifications are based on rapid interference measurements and calculations of the quality of frequencies used in a system by all mobile stations and base stations. The measurements of all frequencies can be done in practice in traditional Time Division Multiple Access (TDMA) systems at lower speeds or if orthogonal frequency division multiplexing (OFDM) is used.
Two main practical problems with conventional DFH are the need to perform rapid interference measurements at all relevant frequencies, both at the mobiles and the base stations; and the signaling overhead required to communicate the measurement results to the base station.
Using real time inter-base signaling for inter-cell interference management and taking advantage of frame synchronization on a system level, an alternative and practical version of DFH finds a solution for these bottlenecks. This technique is called Dynamic Frequency Hopping with Network Assisted Resource Allocation (DFH with NARA). The feature of this technique is that it benefits from frame synchronization on a system level and provides functionality identical to that of the measurement-based DFH.
FIG. 1 illustrates a conventional system structure, where NARA is used for downlink DFH implementation. The system of FIG. 1 includes a conventional mobile station (MS) and base station (BS), with some added functionality. At the MS, the additional functions include pathloss measurements (10) on the frequencies of interest, transmission (12) of the measurement results and the use (14) of a specified FH pattern assigned by the BS. At the BS, the additional functions include the collection (20) of all measurements from the MSs within the BS coverage area, obtaining (22) a local copy of measurements from all MSa at all BSs, processing (24) these measurements along with copies (26) of the existing FH patterns from all BSs in order to identify FH patterns for the given BS, transmitting (28) the FH patterns or the incremental changes in these patterns to the MSs.
FIGS. 2-4 show exemplary operations of FIG. 1. In FIG. 4, PUE-i represents a FH pattern assigned to the ith MS.
Referring to FIGS. 2-4, the system of FIG. 1 operates in the following manner:
Each user (MS) measures pathlosses (10) to the neighboring BSs and transmits (12) this information to its serving BS on a regular basis as show in FIG. 2. The measurement reporting rate in DFH with NARA need not be very high, e.g., the rate used for Mobile Assisted Handoff would be enough.
Each BS communicates to several tiers of its neighbour BSs the information about its own resource utilization: time slots, frequency hopping patterns, and power levels that are currently in use as shown in FIG. 3.
Combining the information received from other BSs regarding to their own resource utilization and the pathloss measurements reported by its MSs, the serving BS calculates the interference level at each available resource, then determines the least-interfered time slot and FH pattern pair, and finally assigns this to the MS as shown in FIG. 4.
In this instance the mobiles are not assigned a pre-defined pattern (such as pseudo random or cyclic hopping patterns). The hopping sequence is generated by the BS dynamically according to the interference level on each frequency at each hop. The BS may communicate the entire hopping sequence or only the incremental changes in the frequencies within the hopping sequence to the MSs.
This procedure applies to new as well as to currently active users; the serving BS continuously monitors each user's performance and reassigns it a new set of resources if the performance degrades below a threshold.
Although DFH improves the performance compared to conventional systems as well as systems using CFH, it requires BS coordination. In conventional wireless communications systems, neighboring BSs do not have a wired link between each other. Therefore, exploiting DFH in the current conventional wireless communications systems encounters the same practicality bottleneck described above.
As is apparent to one skilled in the art, integrating relaying concepts into conventional wireless communications systems increases high data rate coverage as well as capacity in a cost-effective manner. However, due to the bottleneck described above, conventional DFH may reduce the benefit of deploying relays.
In view of the above, there is a need for an improved system and method for resource management in relay networks.